1. Field of the Invention
The present invention relates to high-frequency-signal switching circuits, and more particularly, to a high-frequency-signal switching circuit which is connected to the input end of a TV-set tuner, which can be switched between when a strong electric field is input and when a weak electric field is input, and which has a reduced high-frequency-signal transfer loss and a reduced number of necessary components.
2. Description of the Related Art
There has been known a high-frequency-signal switching circuit connected to the input end of a TV-set tuner in order to handle both a strong-electric-field state and a weak-electric-field state. The high-frequency-signal switching circuit has a first high-frequency-signal path in which a high-frequency signal is transferred through a high-frequency amplifier stage, and a second high-frequency-signal path in which a high-frequency signal bypasses the high-frequency amplifier stage. In a weak-electric field state, a high-frequency signal is transferred through the first high-frequency-signal path, and is amplified by the high-frequency amplifier stage such that the high-frequency signal has a predetermined level when input to the TV-set tuner. In a strong-electric field state, a high-frequency signal is transferred through the second high-frequency-signal path so as to bypass the high-frequency amplifier stage, so that the high-frequency signal does not exceed the predetermined level when input to the TV-set tuner.
FIG. 3 is a circuit diagram of a conventional high-frequency-signal switching circuit. A tuner in a TV set is also shown in the figure.
As shown in FIG. 3, the high-frequency-signal switching circuit 30 is formed of a first high-frequency-signal path 31, a second high-frequency-signal path 32, an input coupling circuit 33, a high-frequency-signal input terminal 34, a high-frequency-signal output terminal 35, a band decoder (switching-voltage supply section) 36, and a power-supply terminal 37.
The first high-frequency-signal path 31 is formed of an amplification field-effect transistor (FET) 311, a first diode 312, a second diode 313, bias-voltage setting resistors 314 and 315, a source resistor 316, a bypass capacitor 317, a load inductor 318, a load resistor 319, bypass capacitors 3110 and 3113, DC-blocking capacitors 3111 and 3116, and bias-voltage setting resistors 3112, 3114, and 3115. The second high-frequency-signal path 32 is formed of a signal-transfer field-effect transistor (FET) 321, a third diode 322, DC blocking capacitors 323 and 326, and bias-voltage setting resistors 324 and 325. The input coupling circuit 33 is formed of inductors 331 and 333, and capacitors 332, 334, and 335.
In the first high-frequency-signal path 31, the gate of the amplification field-effect transistor 311 is connected to the anode of the first diode 312 and to one end of the bias-voltage setting resistor 315, the source thereof is connected to one end of the source resistor 316 and to one end of the bypass capacitor 317, and the drain thereof is connected to one end of the load inductor 318 and to one end of the DC-blocking capacitor 3111. The cathode of the first diode 312 is connected to one end of the bias-voltage setting resistor 314 and to one end of the capacitor 335 in the input coupling circuit 33. The anode of the second diode 313 is connected to one end of the bias-voltage setting resistor 3114, and the cathode thereof is connected to one end of the bias-voltage setting resistor 3115 and to one end of the DC-blocking capacitor 3116. The other end of the bias-voltage setting resistor 314 is grounded, and the other end of the bias-voltage setting resistor 315 is connected to an output end of the band decoder 36. The other end of the source resistor 316 and the other end of the bypass capacitor 317 are grounded. The other end of the load inductor 318 is connected to one end of the load resistor 319. The other end of the load resistor 319 is connected to one end of the bypass capacitor 3110 and to the power-supply terminal 37. The other end of the bypass capacitor 3110 is grounded, and the other end of the DC-blocking capacitor 3111 is connected to the other end of the bias-voltage setting resistor 3112 and to the other end of the bias-voltage setting resistor 3114. The other end of the bias-voltage setting resistor 3112 is connected to one end of the bypass capacitor 3113 and to the output end of the band decoder 36. The other end of the bypass capacitor 3113 is grounded, and the other end of the bias-voltage setting resistor 3115 is grounded. The other end of the DC-blocking capacitor 3116 is connected to the high-frequency-signal output terminal 35.
In the second high-frequency-signal path 32, the gate of the signal-transfer field-effect transistor 321 is connected to the anode of the third diode 322 and to one end of the bias-voltage setting resistor 324, the source thereof is connected to one end of the bias-voltage setting resistor 325 and to one end of the DC-blocking capacitor 326, and the drain thereof is connected to one end of the DC-blocking capacitor 323. The cathode of the third diode 322 is grounded, and the other end of the DC-blocking capacitor 323 is connected to the cathode of the first diode 312. The other end of the bias-voltage setting resistor 324 and the other end of the bias-voltage setting resistor 325 are connected to the output end of the band decoder 36. The other end of the DC-blocking capacitor 326 is connected to the cathode of the second diode 313. In the input coupling circuit 33, one end of the inductor 331 is connected to one end of the capacitor 332 and to the high-frequency-signal input terminal 34, and the other end thereof is grounded. The other end of the capacitor 332 is connected to one end of the inductor 333 and to the other end of the capacitor 335, the other end of the inductor 333 is connected to one end of the capacitor 334, and the other end of the capacitor 334 is grounded. The input end of the band decoder 36 is connected to the power-supply terminal 37, and the power-supply terminal 37 is connected to a power-supply terminal 47 of a TV-set tuner 40.
As shown in FIG. 3, the TV-set tuner 40 is formed of a VHF antenna circuit section (VHFANT) 41v, a UHF antenna circuit section (UHFANT) 41u, a VHF high-frequency amplifier section (VHFRFAMP) 42v, a UHF high-frequency amplifier section (UHFRFAMP) 42u, a VHF high-frequency circuit section (VHFRF) 43v, a UHF high-frequency circuit section (UHFRF) 43u, a VHF mixing stage (VHFMIX) 44v, a UHF mixing section (UHFMIX) 44u, an intermediate-frequency amplifier section (IFAMP) 45, an intermediate-frequency-signal output terminal 46, and the power-supply terminal 47.
In this case, the input end of the VHF antenna circuit section 41v is connected to the high-frequency-signal output terminal 35 of the high-frequency-signal switching circuit 30, and the output end thereof is connected to the input end of the VHF high-frequency amplifier section 42v. The input end of the UHF antenna circuit section 41u is connected to the high-frequency-signal output terminal 35, and the output end thereof is connected to the input end of the UHF high-frequency amplifier section 42u. The output end of the VHF high-frequency amplifier section 42v is connected to the input end of the VHF high-frequency circuit section 43v, and the output end of the UHF high-frequency amplifier section 42u is connected to the input end of the UHF high-frequency circuit section 43u. The output end of the VHF high-frequency circuit section 43v is connected to the input end of the VHF mixing stage 44v, and the output end of the UHF high-frequency circuit section 43u is connected to the input end of the UHF mixing stage 44u. The output end of the VHF mixing stage 44v is connected to the input end of the intermediate-frequency amplifier section 45, and the output end of the UHF mixing stage 44u is connected to the input end of the intermediate-frequency amplifier section 45. The out put end of the intermediate-frequency amplifier section 45 is connected to the intermediate-frequency-signal output terminal 46.
The high-frequency-signal switching circuit 30 having the above structure operates in the following way.
When the TV-set tuner 40 connected to the high-frequency-signal switching circuit 30 is used in a weak-electric-field area, namely, an area where the field intensity of received signals is low, the band decoder 36 in the high-frequency-signal switching circuit 30 is switched to output a voltage VB, such as 5 V, equal to a power-supply voltage from its output end. Then, the voltage VB output from the band decoder 36 is sent to the gate of the amplification field-effect transistor 311 through the bias-voltage setting resistor 315 to make the amplification field-effect transistor 311 be in an operation state. At the same time, current caused by the voltage VB flows through the bias-voltage setting resistor 315, the first diode 312, and the bias-voltage setting resistor 314 into the ground to turn on the first diode 312. Current caused by the voltage VB also flows through the bias-voltage setting resistors 3112 and 3114, the second diode 313, and the bias-voltage setting resistor 3115 into the ground to turn on the second diode 313. Current caused by the voltage VB also flows through the bias-voltage setting resistor 324 and the third diode 322 to make the gate voltage of the signal-transfer field-effect transistor 321 lower than the source voltage thereof to turn off the signal-transfer field-effect transistor 321.
Therefore, whereas the first high-frequency-signal path 31 is active, the second high-frequency-signal path 32 is inactive. A low-level high-frequency signal input to the high-frequency-signal input terminal 34 is sent through the first diode 312, which is on, to the amplification field-effect transistor 311, is amplified to a predetermined level by the amplification field-effect transistor 311, and then, is sent through the second diode 313, which is on, to the high-frequency-signal output terminal 35. At this point of time, since the signal-transfer field-effect transistor 321 is off, the high-frequency signal is not sent through the signal-transfer field-effect transistor 321 to the high-frequency-signal output terminal 35.
When the TV-set tuner 40 connected to the high-frequency-signal switching circuit 30 is used in a strong-electric-field area, namely, an area where the field intensity of received signals is high, the band decoder 36 in the high-frequency-signal switching circuit 30 is switched to output a voltage VE, such as 0 V, equal to a ground voltage from its output end. Then, even when the ground voltage VE output from the band decoder 36 is sent to the gate of the amplification field-effect transistor 311 through the bias-voltage setting resistor 315, it cannot make the amplification field-effect transistor 311 be in an operation state. The amplification field-effect transistor 311 is in an non-operation state. At the same time, current does not flow through the bias-voltage setting resistor 315, the first diode 312, and the bias-voltage setting resistor 314 into the ground, by the ground voltage VE, so that the first diode 312 is turned off. In the same way, current does not flow through the bias-voltage setting resistors 3112 and 3114, the second diode 313, and the bias-voltage setting resistor 315 into the ground, by the ground voltage VE, so that the second diode 313 is also turned off. Current does not flow through the bias-voltage setting resistors 324 and the third diode 322, by the ground voltage VE, so that the voltage difference between the gate and the source of the signal-transfer field-effect transistor 321 becomes zero to turn on the signal-transfer field-effect transistor 321.
Therefore, whereas the first high-frequency-signal path 31 is inactive, the second high-frequency-signal path 32 is active. A high-level high-frequency signal input to the high-frequency-signal input terminal 34 is sent through the signal-transfer field-effect transistor 321, which is on, to the high-frequency-signal output terminal 35. At this point of time, since the first diode 312 and the second diode 313 are both off, and the amplification field-effect transistor 311 is inactive, the high-frequency signal is not sent through the amplification field-effect transistor 311 to the high-frequency-signal output terminal 35.
Then, the high-frequency signal sent to the high-frequency-signal output terminal 35 is sent to the TV-set tuner. If the high-frequency signal is a received VHF-band TV signal, signal components in unnecessary signal-frequency bands are removed from the received TV signal by the VHF antenna circuit section 41v, the resultant signal is amplified to a predetermined level by the VHF high-frequency amplifier section 42v, signal components in unnecessary signal-frequency bands are again removed from the amplified signal by the VHF high-frequency circuit section 43v, the resultant signal is converted to an intermediate-frequency signal by the VHF mixing stage 44v, the obtained intermediate-frequency signal is amplified to a predetermined level by the intermediate-frequency amplifier section 45, and the amplified signal is sent to the intermediate-frequency-signal output terminal 46. If the high-frequency signal is a received UHF-band TV signal, signal components in unnecessary signal-frequency bands are removed from the received TV signal by the UHF antenna circuit section 41u, the resultant signal is amplified to a predetermined level by the UHF high-frequency amplifier section 42u, signal components in unnecessary signal-frequency bands are again removed from the amplified signal by the UHF high-frequency circuit section 43u, the resultant signal is converted to an intermediate-frequency signal by the UHF mixing stage 44u, the obtained intermediate-frequency signal is amplified to a predetermined level by the intermediate-frequency amplifier section 45, and the amplified signal is sent to the intermediate-frequency-signal output terminal 46.
In the known high-frequency-signal switching circuit 30, when the first high-frequency-signal path 31 becomes inactive and at the same time, the second high-frequency-signal path 32 becomes active, a high-frequency signal is transferred in the second high-frequency-signal path 32 through the signal-transfer field-effect transistor 321, which is on. Therefore, a signal transfer loss, for example, of about 3 dB to 4 dB occurs due to the signal-transfer field-effect transistor 321. In addition, since relatively expensive circuit components, such as the high-frequency field-effect transistor 321, are required to make the second high-frequency-signal path 32, the manufacturing cost of the high-frequency-signal switching circuit 30 becomes high.
In the known high-frequency-signal switching circuit 30, when the first high-frequency-signal path 31 is inactive, an off bias voltage which makes the first diode 312 and the second diode 313 off is relatively shallow. Therefore, when a high-level high-frequency signal is sent, a part of the high-level high-frequency signal flows into the first diode 312 and the second diode 313, the high-frequency signal which is transferred through the second high-frequency-signal path 32 may be distorted.